The increasing development in the use of electromagnetic waves in fields as diverse as telecommunications, medical applications, radar, . . . , leads to implementation techniques being varied firstly to control wave propagation and secondly to control wave radiation. In both cases, the means implemented are defined not only by the general radio characteristics required: frequency bands; power requirements; admissible losses; interconnection coplexity; and mission in the broad sense of the term; but also by a set of other criteria that are not specifically concerned with radio, including parameters such as mass, circuit volume, or acceptable temperature range over which the technology used must be capable of operating. These additional criteria also come under the heading "mission in the broad sense of the term". The particular technology chosen must satisfy both radio criteria and criteria that are mechanical, structural, and thermal.
It will readily be understood that environmental and installation conditions differ for microwave equipment depending on whether it is installed on board a satellite, an aircraft, or a submarine, for example, and that this has an impact on the way the technology required for making the equipment is defined and selected.
There is no doubt that the best known means for conveying an electromagnetic wave is a hollow tube. It may be simple in shape being rectangular or circular in section or it may be more complicated, e.g. being hexagonal in section. The applicable range of frequencies is very wide, running from a few gigahertz to several hundred gigahertz, i.e. from centimetric waves to submillimetric waves. Below a few megahertz, waveguides are difficult to use because of their size and mass. Other types of propagation are then used.
A non-exhaustive list includes the following:
coaxial lines and derivatives thereof;
three-plate lines, and
microstrip lines and derivatives thereof.
These various means are in widespread use for propagating signals from DC to a few tens of gigahertz. Put simply, it may be said that radio properties (impedance, propagation constant, etc. . . . ) result from the positioning of two conductors relative to each other by means of a support material or a dielectric spacer. In practice, the materials commonly used have dielectric constants lying in the range 1 to 10, and they may be as much as 40 in some applications.
So far as radiation is concerned, radiating elements have appeared over the last ten years which are remarkable both as to their simplicity of manufacture and as to their characteristics of lightness and ability to be shaped. These elements are printed antennas ("patches") based on using a resonant element etched on a dielectric support, with the assembly being implanted on a ground plane. Here again, these concepts make it possible to propose solutions which are very competitive in terms of volume, compactness, and mass.
These two centers of interest (making circuits, and making the radiating elements) have led manufacturers to offer an ever increasing range of dielectric materials having wider and wider fields of application.
The constraints for utilization in the space environment are well known and generally bear on:
equipment mass;
temperature ranges and thermal stresses;
vibration levels; and
physical stability in a vacuum (no degassing).
The object of the invention is to provide substrates of variable permittivity.